1. Field of the Invention
The present invention belongs to the field of metallurgical technology, in particular relates to a method for manufacturing hollow ingot for retaining ring of large generator by electroslag remelting.
2. The Prior Arts
It is determined by the constitution of primary energy source in China that coal-fired power plants are predominant, and this is the biggest challenge for China in controlling greenhouse gas emissions. Therefore, using less coal to generate more electric power and using coal in a clearer and more effective way have become the strategic issues related to the sustainable development of China's national economy, wherein one of the most important ways is to develop supercritical and ultra supercritical generating units with high capacity and high parameters so as to increase the thermal efficiency of the generating units. Retaining ring of a steam turbine generator is a critical component for thermal power generator and is a ring-shaped forging used for fastening winding coils on both ends of a generator rotor. The retaining ring is also used for preventing the coils on both ends of a driving motor rotor from flying off under centrifugal action. The retaining ring works in the environmental conditions of high speed, high stress, corrosion and high magnetic field, therefore, the quality of the retaining ring is in direct relation to the safe operation of the whole generating unit, and the retaining ring is thus one of the most critical components of a steam turbine generator unit. Meanwhile, the higher the capacity of the generating unit is, the larger the size of the retaining ring is required.
The traditional manufacture method for retaining ring is to use a solid ingot as forging material, and the solid ingot needs to be processed into a hollow ingot first. The manufacture process comprises the steps of manufacturing a solid ingot through electric furnace melting and electroslag remelting; heating the solid ingot to a forgeable temperature range; upsetting and punching with a 10,000-ton open forging press to make the solid ingot into a hollow ingot with a certain inside diameter; inserting a mandril into the hollow ingot; and stretching the hollow ingot while hole expansion. In order to ensure the interior quality of the ingot, the forging deformation at each time needs to be slight. However, the ingot is extremely prone to cracking during repeated-heating slight-deformation free forging. Forging has to be stopped at any time when cracks are found, and the cracks can only be removed after the ingot is cooled. After the cracks are removed, the ingot needs to be heated to the forgeable temperature range again to proceed with the forging, and the preparation of the hollow ingot is not completed until the inside and outside diameters both reach required dimensions.
The traditional method for manufacturing hollow ingot for retaining ring has complex process and low forging yield, and a large amount of material is wasted during punching process. As multiple heating and deformation processes are used, the internal structure of the ingot is prone to change, and forging cracks are prone to form but difficult to remove. Repeated cooling and heating have a serious impact on the quality of the product, thus it is difficult to prepare hollow ingot for supersized retaining ring, and product accuracy and material uniformity cannot be ensured.